Multi-scale analysis reveals vegetation buffers human disturbance impacts on urban bird functional diversity

Functional traits determine how urban bird communities respond to the novel environmental conditions in cities. While human disturbance generally has a negative impact on functional diversity, the potential of vegetation structures in buffering these effects, and the spatial scale at which these processed occur, remain poorly understood. In this study, we aim to analyse both the spatial scale of effect and the indirect effects of vegetation on functional diversity of urban birds. We estimated functional diversity —richness, evenness, and divergence— within a community of 95 breeding bird species, and examined how these metrics are influenced by human disturbance at three spatial scales: the monitoring grid, the centroids of bird territories, and the precise point locations of the birds. Our findings reveal that while human disturbance remains the primary driver of changes in bird functional diversity, the strength and nature of the effects are scale specific. Specifically, human population density exhibits a stronger negative impact at larger spatial scales, leading to a decrease in functional diversity, while higher noise levels are positively associated with functional diversity, particularly at finer spatial scales. The positive effect of noise at fine scales is probably driven by the unique conditions in urban environments, where noisy areas often coincide with lower human presence, creating opportunities for certain bird species when associated to complex vegetation structures. These findings underscore the need to consider multiple spatial scales to comprehensively understand the drivers of functional diversity in urban bird communities, and highlight the importance of vegetation structure in buffering disturbance effects.